Various types of exposure apparatuses are used to manufacture microdevices such as ICs and LSIs. The exposure apparatus has an exposure light source, and exposes a photoresist-coated wafer to a pattern (e.g., a circuit pattern) formed on a master such as a mask or reticle with light generated by the exposure light source.
For a higher integration degree of microdevices, the wavelength of the exposure light source must be shortened. As one of short-wavelength exposure light source candidates, an X-ray light source has been proposed and developed. Of such light sources, an X-ray point source which is smaller in size and lower in cost than a synchrotron, and a stand-alone light source used as an EUV (Extreme UltraViolet) light source receives a great deal of attention. These light sources generate X-rays and EUV light whose intensities change like pulses.
In order to realize a high throughput in X-ray exposure and EUV exposure, the intensity of light to be output and the sensitivity of the resist must be increased. The intensity per pulse is being increased for obtaining high-intensity pulse X-rays or pulse EUV light. However, the pulse intensity and pulse rate basically have a trade-off relationship, and room is left for improvements of both the pulse intensity and pulse rate.
Exposure of a substrate such as a wafer requires high-accuracy exposure amount control. For example, in many cases, the line width accuracy in pattern exposure at a minimum line width of 100 nm or less must be equal to or lower than ±10% of the minimum line width. One of the factors which decrease the line width accuracy is the exposure amount error. The exposure amount accuracy depends on the error budget and the processing step, and is desirably some tenths of a percentage.
The pulse X-ray intensity of the current pulse X-ray source is known to suffer from a large dispersion. For example, as shown in FIG. 3, the dispersion at 3σ reaches several tens percent. Even if the cumulative exposure amount is measured, the intensity of the final pulse is adjusted, and light is emitted with a given target value, a predetermined cumulative exposure amount accuracy cannot be obtained due to the dispersion in the final pulse. This problem becomes more serious as the number of exposure pulses per shot decreases by increasing the pulse intensity per pulse.
Let i be the average of the X-ray intensity per pulse, and D be the exposure amount target value of one shot. In this case, an average number N of pulses per shot is given by:N=D/i. 
Letting S be the dispersion in the X-ray intensity of each pulse at 3σ in generating the number of pulses by a target value, an exposure amount error E of one shot due to the intensity dispersion is given by:E=S/N. 
For S=75% and N=200, E=0.375%. This value may not be so large with respect to the exposure amount error target value (e.g., 0.2%). In practice, the average of an actual pulse intensity at N=200 readily shifts from the average i of the pulse intensity for a sufficient number of samples. The cumulative exposure amount greatly disperses for each shot. That is, the number N of pulses=200 is too small, and the average of the actual pulse intensity may shift from a statistical average.
In general, therefore, the cumulative exposure amount per shot is measured by a measurement device, and when the pulse intensity average at one shot shifts from i, the number of pulses is increased/decreased. In this case, the cumulative amount up to a pulse before the final pulse can be generally detected with sufficient accuracy. However, the intensity of the final pulse disperses, and the cumulative exposure amount can only attain an accuracy:E=S/N. 
In a plasma X-ray source, as shown in FIG. 2, the X-ray conversion efficiency becomes higher for a larger input energy of one pulse. In general, the generation efficiency of the X-ray source is not so high, and an increase in X-ray conversion efficiency is very effective for increasing power. For this reason, X-ray source vendors have developed X-ray sources for a larger input energy per pulse. As a result, it becomes more difficult to increase the cumulative exposure amount, as described above.
In exposure by an excimer laser beam similarly using pulse-like energy, the pulse intensity can be easily decreased by decreasing the laser discharging voltage. The exposure amount accuracy is improved by decreasing the pulse intensity near the final portion in each exposure shot.
It is also proposed to control the pulse X-ray intensity by changing the laser beam intensity of a laser plasma X-ray source, as disclosed in Japanese Patent Laid-Open No. 9-184900.
One of pulse X-ray sources excites X-rays by discharge energy. A so-called dense plasma focus device belongs to this type. This type of source does not use any laser, and the above-mentioned intensity control by a laser beam cannot be adopted.
There is a method of adjusting the X-ray intensity of a discharge pulse X-ray source by the discharging voltage. The adjustment method using the discharging voltage is generally employed in intensity adjustment of a pulse laser beam. This method has various merits such as high response, continuous intensity adjustment, and the absence of any complicated arrangement. When, however, the adjustment method using the discharging voltage is applied to the discharge pulse X-ray source, discharge may become unstable or a generated wavelength may change upon greatly changing the discharging voltage.